1. Field of the Invention
The invention relates to a protection circuit for brushed DC motors.
2. Description of the Related Art
Conventionally, there are two control principles for brushed DC motors, one (as shown in FIG. 1) is that a DC power supply directly supplies power to motor windings through a mechanical contact commutator and carbon brushes and the other one (as shown in FIG. 2) is that an AC power supply supplies power to motor windings through the mechanical contact commutator and carbon brushes after the alternating current is converted into the direct current by a rectifier circuit.
The foregoing brushed DC motors are protected in a simple manner. Specifically, the over-current and over-temperature protection are achieved when the temperature of windings becomes too high after heavy current passes through and the heat will be transferred to a temperature controller, which will function to disconnect the power supply circuit. However, in some areas and for some special DC motors, a motor may be burned out before the temperature controller functions due to various factors such as ambient temperature, heat dissipation speed, and short withstand time for heavy current, etc., therefore the protection function fails. For example, when a water pump works under water, where the working environment is favorable for heat dissipation, after heavy current passes through the windings, the temperature controller will not disconnect the circuit because the temperature is not high enough even if the windings become heated, hence the motor will be burned instantly. As a result, the protection circuit has low reliability and poor protection effects.